Gravity, Space And Orbital Speed (T1) Flashcards
Definite the centre of gravity…
- the point at which the whole weight of an object appears to act
- also sometimes referred to as the centre of mass
How do you find the centre of gravity on a sheet of card or any other uniform material?
Fold the sheet in half as many times as you can and the centre of gravity is located where the axis of symmetry cross
How do you find the centre of gravity on an irregular shape?
- suspend the shape freely by a point on its edge
- hang a plumb line from the pivot point
- draw a line along the plumb line from the pivot point
- repeat the process from different points
- the centre of gravity is the point at which the lines cross
A boy stands in the middle of a beam, stretching across a crevice. Describe what is happening with the upward forces at work?
The upward forces at either end are equal.
If a beam stretches across a river, balancing on each bank, and a boy stands on the right side of the beam, what are the upward forces doing?
All the upward force will be on the right side of the beam.
If a boy stands 1/4 along from the centre of a beam that is balancing across a crevice, where are the upward forces concentrated?
The upward force nearest to him is 3/4 of his height and the upward force on the end furthest from him is 1/4 of his weight.
Describe Newton’s law of gravity…
- between any two objects there is a force of attraction
- the greater the masses, the stronger the gravitational force
- if the distance between the masses increases, the gravitational force decreases
What two main factors affect gravitational force?
- the masses of the two objects
- the distance between the masses
Describe how gravitational forces obey an ‘inverse square law’…
If the distances between the masses is doubled, the forces between them are quartered; if the distance is trebled, the forces become one ninth of what they were
The gravitational pull at the surface of the sun is how many times stronger than the surface of the earth?
More than 30 times stronger
If the earth’s radius increased but it’s mass stayed the same, the gravitational force would increase or decrease? And why?
It would decrease as you would be further away from its centre (larger distances reduce gravitational pull)
Why is the gravitational pull of neutron stars and black holes so immense?
- extremely heavy
- have a very small diameter
What would happen if you dropped a coin on a neutron star from one meter above the ground?
It would be travelling at around half the speed of light by the time it hit the ground.
What does it mean when we say planets are ‘non-luminous objects’?
They do not emit light
Why / how do we see the planets if they do not emit light?
Because they reflect light from the sun
What holds planets in orbit?
The gravitational pull of the sun
What shape are planet’s orbits?
Their orbits are elliptical in shape
Briefly explain how the earth is held in orbit by the sun…
The earth is effectively falling continuously towards the sun, however the earth is also travelling at right angles to that acceleration and the force towards the sun changes the motion of the earth just enough to keep it in a circular path. The sun’s gravity doesn’t change the earth’s speed so the orbit is stable.
What is a satellite?
An object that orbits a planet
What is a moon?
A natural object that orbits a planet.
Our moon is often referred to as the Earth’s satellite.
How far from the Earth is our moon?
Around 400,000 km
What is the mass and surface gravity of the moon relative to the Earth?
The mass and surface gravity of our moon is around 1/6 of the Earth
Why do we only ever see one side of the moon?
- it takes the moon 29.5 days to orbit the Earth
- it also takes the moon 29.5 days for one full rotation on its axis
- as the time taken for both of these events is the same, we always see the same part of the moon’s surface pointing towards the Earth
How many moons does Mars have?
2 moons
How many moons does Uranus have?
27 moons
How many moons does Jupiter have?
Over 60 moons
Explain how the distance from the Earth affects the orbits of different man-made satellites…
- fast moving satellites like spy satellites and the international space station have low orbits, close to the atmosphere, and orbit the Earth every few hours
- weather/communication satellites have high orbits and usually take 24 hours to complete one orbit
Explain what’s meant by a ‘geostationary orbit’…
As weather and communication satellites take 24 hours to complete one orbit of the Earth, it makes them appear stationary in the night sky. This is known as a geostationary orbit.
What do comets orbit?
Comets orbit the sun
What size do comets tend to be and what are they made of?
Comets are around 1-30 km in diameter and made of dust and ice
What shape are comets orbits?
Comets orbits are extremely elliptical in shape and often described as being cigar-shaped
Explain how the shape of a comets orbit affects its speed and appearance
- at times comets are very close to the sun and move at very high speeds
- other times they are found at the outer reaches of the solar system, moving at a much slower rate
- closer to the sun, some of a comets frozen gases evaporate and this forms a long tail that shines in the sunlight
How long does a comet usually take to complete a full orbit?
Most take around 200 years and many take thousands of years to return
What was the first comet ever to be predicted?
- Halley’s Comet
- Edmund Halley was the first person to predict the return of a comet
- Halley’s Comet returns every 76 years
What are asteroids?
Minor planets or rocks that orbit the sun
Where is there a well known asteroid belt in the solar system?
Between Mars and Jupiter
How big are asteroids?
They can vary from a few metres across to hundreds of km
What was the first asteroid to be discovered and how big was it?
The first asteroid to be discovered was called Ceres and had a diameter of 933km
What is the theory between the origin of asteroids?
It is believed they were formed at the same time as the rest of the solar system and may be the rocky remains of a planet that broke apart of failed to form
What was the Ptolemaic System?
- an initial idea that the Earth was at the centre of the universe and all other astronomical bodies moved around it
- it was named after the Egyptian astronomer Ptolemy who wrote about it in approx. 150 AD
What is the Heliocentric System?
- the sun centred model of the solar system
- suggested by Nicolai Copernicus in 1530
What is a galaxy and how do they work?
- gravitational forces between stars cause them to cluster in groups called galaxies
- a typical galaxy is a collection of around 100 billion stars
- they are held together by their mutual gravity and would all collapse inwards if they were not all orbiting around the centre of the Galaxy
How far from the Earth is the sun?
Around 150 million km
What is the surface temperature and core temperature of the sun?
- the surface temperature is around 6000 degrees centigrade
- within its core it can be 15,000,000 degrees centigrade
Describe our galaxy and our position within it…
- out Galaxy is called the Milky Way
- it is a ‘spiral’ Galaxy
- the Earth is around 2/3 of the way out from the centre of our Galaxy along one of the arms of the spiral
How many galaxies are there believed to be in the known universe?
Around 100 billion
The distances between galaxies are ……… of times ……… than the distances between stars
The distances between galaxies are millions of times greater than the distances between stars
The distances between stars in a Galaxy are ……… of times ……… than the distances between planets and the sun
The distances between stars in a Galaxy are millions of times greater than the distances between planets and the sun
How do you calculate orbital speed?
Orbital speed = 2 x pi x orbital radius (r) / time period (T)
2 x pi x r / T
When calculating orbital speed what does T refer to?
The time period for one complete orbit
In orbital terms the distance travelled is…
…the circumference of the orbit
